# RD-E: 2202 Ditching using SPH

The ditching of an object into a pool of water is studied.

The simulation results are compared to the experimental data and the analytical results. The ditching object is modeled as a rigid prism. The water is modeled using SPH elements with an outlet condition to allow for a non-reflecting boundary condition.

## Options and Keywords Used

- SPH modeling
- Hydrodynamic viscous fluid law (/MAT/LAW6 (HYDRO or HYD_VISC)) and impact on water modeling
- Interface /INTER/TYPE7
- Rigid body (/RBODY)
- Initial velocity (/INIVEL)
- Accelerometer (/ACCEL)
- Gravity (/GRAV)
- SPH outlet (/SPH/INOUT)
- Parameter input (/PARAMETER)

## Input Files

## Model Description

The problem consists of a simple object falling into water simulating the ditching of a helicopter.

Units: mm, ms, KN, GPa, kg.

^{2}, also using the experimental data obtained from the Politecnico di Milano.

^{1}

**Material Properties****Value**- Initial density
- 7.8 x 10
^{-6}kg.mm^{-3} - Young's modulus
- 206.9 GPa
- Poisson's ratio
- 0.3

**Material Properties****Value**- Initial density
- 1 x 10
^{-6}kg.mm^{-3} - Kinematic viscosity
- 0 GPa
- Gamma
- 6.1
- P0
- 1e-4 GPa
- Psh
- 0 GPa
- p*
- 0.36885 GPa

The impacting prism is modeled using shell elements with an average mesh size of 15 mm x 15 mm. To shorten the computation, it is made rigid with an accelerometer on the main node of the rigid body. Additional mass is added to the main node of the rigid body, so the total mass of the prism is 27 kg.

The water is modeled using SPH particles with a face-centered cubic mesh and
smoothing length "`ho`" equal to 21 mm. Each SPH weighs m=2.61 g.
The SPH particle mesh can be generated in HyperWorks
using the SPH panel with a face-centered cubic pitch=15, and a material density of
1.0e-6 $\raisebox{1ex}{$kg$}\!\left/ \!\raisebox{-1ex}{$m{m}^{3}$}\right.$. HyperWorks will then
automatically create SPH property /PROP/SPH. This results in
222005 SPH elements. The size of the water is adapted to the shape of the object to
reduce the model’s size and the simulation’s CPU time. For an SPH analysis, air is
not typically modeled.

### Boundary Setup

An initial velocity of 11 m/s in the Z direction is defined on the main node of the rigid body of the prism. Gravity is also applied to the prism.

`Ityp`=3 is defined using a surface at a distance equal to 2 x h

_{o}inside the SPH water elements. The surface is placed at a distance equal to 2 x h

_{o}inside the water. The surface is shown in green in Figure 3 and is oriented so that its normal vector points inside the domain.

The fluid-structure contact between the impacting prism and SPH water is modeled
using a /INTER/TYPE7 sliding penalty contact interface. The
impacting prism is the main surface and the SPH elements are the secondary nodes.
The contact thickness gap is defined as `Gap`_{min}=3. For situations when the contact thickness gap is important such as when the
SPH particles are in a tank, define `Gap`_{min}=1/2 * (shell thickness + SPH diameter) or for solid elements `Gap`_{min}=1/2 * (SPH diameter).

## Results

The SPH particles are supported as MASS elements in HyperView. Their display type and size can be set in HyperView by using . In this example, the animation results are directly output to an h3d file using the /H3D option. The pressure in the water can be plotted.

### Output Acceleration

^{1}and the analytic solution proposed by Von Karman.

^{2}The simulation acceleration is filtered using the same CFC 60 (-3db) filter frequency as the test data was filtered.

The SPH results are very close to the experimental test results and also to the analytical solution. The simulations show that the SPH approach using the OUTLET option /SPH/INOUT correctly models a vertical ditching event without any numerical problems.

^{1}CAST Deliverable 5.5.1 Generic Water Impact Tests performed at Politecnico di Milano (Polytechnic University of Milan)

^{2}Olivier Pastore Study and modelization of rigid bodies impact during sea landing phase; Annex 1 Von Karman's Theoretical Models, T. Miloh et al. May